Meteorology

Wind

Meteorology or the weather and especially the wind have a major impact on the safety. Wind is air flowing from a high-pressure area to a low-pressure are. The wind has a deviation to the right in the Northern Hemisphere. This is due to the earth’s rotation and is called: coriolis force. Due to the coriolis force, the wind does not flow in a straight line from the high to the low, but circles clockwise out of the high pressure area and then counterclockwise into the centre of the low pressure area. In this process, the wind makes an angle of about 15 degrees with the isobars (depending on the roughness of the earth’s surface).

Coriolis force

The deviation to the right (coriolis force) occurs because the earth rotates around its axis in 24 hours. If we look at the earth from above (we are looking at the north pole) the earth rotates counterclockwise. But if we look at the earth from the side, the earth rotates to the right. The speed to the right of, say, the parallel of 45°N is much higher than that of 50°N. Indeed, both go around in 24 hours, but the length of the parallel of 50°N is smaller than that of 45°N. Air flowing from a high-pressure area at 45°N to a low-pressure area at 50°N retains its higher velocity to the right, thus causing the deviation to the right.

Prevailing winds

The image below shows the prevailing winds on Earth.

So if we now look at a weather map, we can work out for ourselves what the wind direction is in different places. For example at the location of the green cross, the wind direction is like the red arrow, namely from high to low with a deviation to the right. Note that the weather map below is not a mercator chart. So north is parallel to the meridians and not straight up, as the green arrow also indicates. The wind direction, is approximately southwest, indicated by the red arrow. If the isobars are close together it blows hard, if the isobars are far apart there is less wind.

Weather map reading exercise:

Download the current weather map and estimate wind direction and strength in the Netherlands.
Check your estimate against the weather forecast.

Buys Ballot’s law

This law implies that if you stand with your nose in the wind, the low is to the right behind and the high to the left in front of you. For example, imagine standing between the high and the low in the drawing below with your nose in the wind:

Beaufort scale

The Beaufort scale runs from wind force 1 to 12. in NL, warnings are issued from wind force 6.

We can calculate back and forth with the following rules of thumb:

From knots to beaufort:
(knots/5)+1 bft
+1 bft up to 40 knots, after that it’s enough.

From m/s to knots:
m/s x 2 = knots

Backing – Veering

The wind can veer that is, it rotates clockwise, so e.g. from north to east. The wind backs when it turns anti-clockwise, so e.g. from west to south.

Weather maps

On weather maps, we see lines connecting points of equal atmospheric pressure (millibars). We call these lines isobars. When the lines are close together, the difference in pressure is relatively large. As a result, the wind will be strong. If lines are further apart, the pressure difference (the gradient) is low and the wind will not be strong.

Cells

You can classify air by the cell it comes from: Arctic cell, polar cell or tropical cell. If the air comes from land, the air is called continental. Continental air is dry and very hot or very cold. If the air comes from the sea, it is called maritime air. Maritime air is moist and moderate in temperature. Combinations are: Maritime Polar air, Continental Polar air, Maritime Tropical air, Continental Tropical air.

Thunderstorms

Thunderstorms are caused by two layers of air with different static charges sliding over each other. A thunderstorm cloud often comes against the wind you feel at sea level. Leave the water during thunderstorms if you can. Stay in the cabin, away from the mast. Before the thunderstorm starts, you can ground the mast by e.g. running jumper cables into the water via the stays. Do not go swimming.

Geostrophic wind scale

To estimate wind speed, we can use the “Geostrophic wind scale”:

• • On the weather map at your location, measure the shortest distance between two isobars with a map compass.

• • Check what latitude you are at.

• • Read the geostrophic wind from the table.

• • Multiply this wind by a factor to determine the wind at sea level, depending on whether you are on land or at sea, among other things.

• • The curvature of the isobars also determines wind speed. For a high-pressure area, the more curved the stronger the wind. For a low-pressure area, the more curved the looser the wind.

Exercise weather map reading part 2

Download the current Metoffice weather map and determine the wind direction and speed at a number of locations using the “Geostrophic wind scale” in the top left corner and the table below.

Check your estimate against the written-out weather forecast.

development of depression

On the Atlantic Ocean (the “kitchen” of Dutch weather), warm (south) and cold (north) air rub past each other. The cold air has a higher density and a wave is created in the three-dimensional front. The relatively warm air hangs over the cold air to the north. This wave develops into a depression or low-pressure area with a warm front (recognizable by a line of dots), cold front (recognizable by a line of triangles) and a warm sector in between. Because the cold air is heavier, it overtakes the warm air, creating an occlusion front (recognizable by a line with dots and triangles). Due to the prevailing winds, the direction of migration in Europe is always eastwards.

A heat front always announces itself with cirrus clouds, which are wind feathers (ice crystals at high altitudes). This is followed by increasingly lower-hanging greyish clouds with moderate rain.

Ahead of the warm front

A heat front announces itself with cirrus clouds (tall ice crystals) as the warm moist air creeps up over the cold air, thus cooling and condensing. Just before the front passes, winds will briefly contract.

Cross-sectional warm front

During the passage of the warm front, the wind will veer from south-east to south-west. The temperature will rise because after the heat front, you are in the warm sector. Air pressure will drop as you move closer to the center of the low-pressure area.

Ahead of the cold front

After the warm sector comes the cold front.

Cross-sectional cold front

After the passage, the wind will widen from south-west to north-west. Temperatures will drop as you re-enter relatively cold air. The cold front contains Cumulonimbus. From this will come heavy rain and hail showers, thunderstorms and wind gusts. After the passage of the cold front, the weather pattern is showery.

Behind the frontal depression is often a trough. This is an area where the isobars are closer together, so the wind is stronger. This is in contrast to a vore, which is an area where the isobars are far apart, making it less windy.

Sea Wind

Sea breezes and land breezes result from difference in temperature between land and sea. During the day, air over land rises because land warms up faster. That “vacuum” (thermal low) is replenished by air from the sea, which is therefore the sea breeze. At night, the land cools faster than the sea and therefore the air over land drops and the air over the warm sea rises. The “vacuum” at sea is replenished by air from land, thus causing the land wind.

The rising air over land condenses and a sea wind front forms.

When wind or current flows around a land cape, this can greatly increase the wind. This is what we call cape effects. When wind or current is squeezed through a funnel, the funnel effect occurs, there is also an acceleration of current or wind.

Grib files

On board, grib files are useful because they are small files to be retrieved. These can be read into a grib reader such as https://opengribs.org/en/. If you have better internet coverage via phone or WIFI in port, www.windy.com is a good alternative.

Fog at sea

At less than 1000m visibility, we speak of fog. Dense fog at less than 200m. Very dense fog at 50 meters or less. Fog occurs because air with a certain humidity cools and condenses or air with a certain temperature becomes moister, causing condensation. Warming or extraction of moisture allows the fog to dissipate.

Fog (condensation) actually occurs due to two causes: mixing of relatively warm moist air with cold air or due to a cold surface.

Fog by blending

Arctic fog

This is a type of fog that forms when very cold (arctic) air flows out over relatively warm seawater or meets the warm Gulf Stream. This can occur in autumn, for example, when the sea is still relatively warm, with a cold north wind.

Rain fog or frontal fog

This mist forms when rain in a heat front, falls through cold air on a cold surface. Similar to taking a hot shower in a cold bathroom.

Fog due to cold ground

Radiation fog

This fog forms over land due to vertical air movement, convection or radiation in clear skies and low winds (6 to 10 kts), which cools the earth’s surface sharply at night. The offshore land wind at night then blows this fog out to sea. This is especially common near river mouths, as the cold air sinks from land to the lower and relatively warmer river.

Advective fog

This sea fog or sea flare occurs over the sea. In spring, this fog occurs off the Dutch coast when warm moist south-westerly winds sweep across the (then still) cold North Sea. On the east coast of Britain, sea fog then occurs with an onshore, easterly wind.

500MB altitude maps and the jet stream (Jetstream)

If a true meteorologist should choose only 1 map to predict the weather, it would probably be the 500MB altitude map. These 500MB altitude maps show contour lines (so no isobars!) connecting points where the pressure is 500MB. 500MB was chosen because it is about half the air pressure at sea level. Note that the last 0 is always omitted. So 575 means that the air pressure at 5750m altitude is 500MB. The contour lines vary between 4,980 and 6,000 meters. Where the altitude lines are close together, the temperature difference is also large. Cold air has a much higher density than warm air so the 500MB contour lines are higher in warm areas. So where the altitude lines are close together, the temperature difference will be large and the jet stream will blow bringing frontal depressions. The jet stream is thousands of km long, hundreds of km wide, blows at 9 to 10 km altitude and comes from the west. The jet stream blows at up to 11Bft or more, sometimes 350 km per hour. Because the jet stream is similar to a meandering river, it can carry warm air from southern directions but also cold air from northern directions.

Drawing your own weather maps

Since we are very unlikely to have an internet connection during a trip to England or Scandinavia, we cannot easily download weather charts. However, we can draw our own weather charts from a weather message received on VHF, NAVTEX or ship’s radio to better understand the weather situation. The ship’s radio has a long range and the message is extensive and is therefore preferred. With this skill, you can have a much better idea of the weather situation during your sailing trips based on a spoken weather message received on the radio.

Symbols in the weather map

Exercise

1. 1. Record the weather forecast with a voice recorder so you can listen to it after, as it may go too fast. You can also listen to the latest Shipping Forecast on BBC Radio 4. You can also listen to der Seewetterbericht from the Deutscher Wetterdienst online.

1. 1. Enter the schedule. For example, the schedule “Shipping Forecast record” from the Reeds Nautical Almanac. If you listen to the Deutsche Wetterdienst, use the Bordwetterkarte No 09. Here it is very important to use abbreviations and symbols, it is too fast to write out the message completely. So you need to know those symbols and abbreviations then.

1. 1. Enter your notes, in the map such as “UK Shipping forecast areas” from the Reeds NA or the “Bordwetterkarte 9”. In the different areas, note the weather data, such as: the centers of high and low pressure areas, the atmospheric pressure, the wind direction as wind springs, precipitation and visibility (and therefore fronts), the isobars per 4 or 5 Mb (from the wind we can deduce how the isobars should run), etc.

1. 1. You can check your self-constructed weather map with that of the MetOffice: Weather map MetOffice or the weather map on the website of the Deutscher Wetterdienst.